I dont really think there is a method of doing this accurately without identifying the user explicitly.

I would depend on your market as to wich method you would use. For example, if your market is highly technical then perhaps cookies would be a bad idea as the techies may turn them off. OTOH if you market is maily dialup, you may get lots of incorrect stats based on that. (I havent even mentioned proxies and invisible proxies)

I guess it depends on your market and which on is the lesser of twee evils for you.

As merlyn mentions you could generate a gif and have the user type that in along with their vote. You may get a smaller number of votes, (as its a pita to enter) but better quality - it also doesnt prevent double voting.